Echo suppressing circuit



Oct. 7, 1941. D. MITCHELL 2,257,806

ECHO SUPPRESSING CIRCUIT Filed NOV. 22, 1940 2 Sheets-Sheet 1 //v lENTOR By D. M/ 7'CHE L L T0 WEST SUBS AITORNEV 0a. 7; 1941. D, MITCHELLI 2,257,806

ECHO SUPPRESSING CIRCUIT Filed Nov. 22, 1940 2 Sheets-Sheet 2 /A;VEN7'ORBy 0.M/7'CHELLY ALE ENE V Patented Oct. 7,

2,257,806 nono SUPPRESSING CIRCUIT Doren Mitchell, Bound Brook, N. J.,assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y.,a corporation of New York Application November 22, 1940, Serial No.366,511

11 Claims.

The invention relates to two-way telephone systems and particularly tothe switching circuits employed in such systems for suppressing echoesand preventing singing.

The type of echo suppressor most commonly employed in commercial two-waytelephone systems operates in response to signal transmission in atransmission path for one direction to insert a loss in the transmissionpath for the opposite direction, which is of fixed value andsufiiciently large to reduce echoes in the latter path to anunobjectionable amount for the worst echo condition which might existregardless of Whether the echo is strong or weak. In another type ofecho suppressor the value of the loss inserted in the echo path by itsoperation is proportional to the magnitude of the talkers voice wavescontrolling the echo suppressor. The amount of echo from any telephoneconnection depends to a considerable extent on the return loss at eachend of the telephone circuit. If the return loss is high, it is notnecessary to put in as much loss to the echo as if the return loss werelower. An object of the present invention is to automatically adjust theloss inserted by an echo suppressor so that it is never greater than isnecessary to properly suppress the echo, thereby providing improvedoperation particularly from the standpoint of break-ins.

This object is accomplished in accordance with the invention by acircuitarrangement operating under control of the voice currents toadjust the suppression loss in the echo path according to the amount ofreturn loss, so that the totalloss to the echo, including the two-Wirereturn loss, is 'always the same.

In one embodiment this circuit arrangement comprises at each terminal ofa four-wire telephone circuit, a receiving terminal type of echosuppressor with a delayed action transmitting disabler in combinationwith a difierential circuit which automatically compares the receivedspeech waves with the echoes during the time in which the echosuppressor is operated, and adjusts the loss pad which is inserted inthe echo path by operation of the echo suppressor to increase ordecrease the suppression loss according to whether the echo is toostrong or too weak relative to the received speech waves.

The various objects and features of the invention will be betterunderstood from the following detailed description when read inconjunction with the accompanying drawings in which:

Fig. 1 shows a skeleton diagram of a four-wire telephone circuit with anecho suppressor in accordance with the invention applied to bothterminals; and r Fig. 2 shows schematically in more detail the echosuppressor circuits at one terminal of the four-wire telephone circuitof Fig. 1.

In the skeleton diagram of Fig. 1, each of the single lines represents atwo-wire transmission path. Contacting arrow-heads at a point in a lineindicate that the transmission path is made at that point, and separatedarrowheads at a point in a line indicate that the transmission path isbroken or disabled at that point. An arrow pointed from a box towards amake point in a transmission path indicates that the transmission pathwill be disabled at that point in response to the output currents of thetransmission device represented by the box, and an arrow pointing from abox to a break point in a transmission path indicates that thetransmission path will be enabled at that point in response to theoutput current of the transmission device represented by the box. Eachbox containing an armature, connected by an arrow to a point on avariable loss device indicated for convenience as a potentiometerresistance in a transmission path, and two oppositely pointedarrowheads. pointing to the armature, respectively connected todifferent contacts of a differential relay, represents a'two-way relaystepping device operating to adjust the potentiometer resistance in onedirection or the other depending on the direction of operation of thedifferential relay.

The functions of the other transmission apparatus in the diagram,represented by boxes, are indicated by explanatory legends.

The four-wire telephone circuit of Fig. 1 comprises a west-to-eastone-way transmission path EA and an east-to-west one-way transmissionpath WA coupled at the west terminal by the hybrid transformer HW andassociated balancing network NW to a two-way line or circuit LW leadingto a west subscriber's telephone circuit (not shown) and coupled at theeast terminal by the hybrid transformer HE and associated balancingnetwork NE to atwo-way telephone line or circuit leading to an eastsubscriber's telephone set (not shown). The path EA includes at. thewest terminal a normally short-circuited potentiometer resistance P1 andthe amplifier A1, and at the east terminal the amplifier A2. The path WAincludes at the east terminal the normally short-circuited resistance P2and the amplifier A3, and at the west terminal the amplifier A4. t

The transmitting switching control circuit TSw a receivin portion RSE.

of the echo suppressor at the west terminal includes the amplifier A5having its input bridged across the outgoing portion of the path EA onthe output side of amplifier A1. The output of the amplifier A5 connectsthrough the band-pass filter F1 and the wave detector D1 to two parallelcontrol branches, one comprising the upper operating winding 101 of thedifferential relay DRl, and the other, in order, the delay circuit DCr,the hang-over network HN1 and the switching control 61. V

The receiving switching control RSwof the echo suppressor at the westterminal includes the amplifier A6 having its input bridged across the,in-

coming portion of the path. WA in front ofar'nplifier A4. The output ofamplifier AG connects through the band-pass filter F2 and -th e wavedetector D2 to the lower operating winding w; of the differential relayDRI, poled to oppose the operating windingwi of that relay. Theooutputof amplifier A5 also connects through, w ave 'detector D2 and thehang-over network HNz in parallel to the four (switching controls 111,hi, c1 and 11. The differential'relay DRi controls the two-way steppingswitch SS1 which controls the adjustment of the variable loss ,P1..

Similarly, at the east terminal the echo suppressor comprises atransmittingportion TSE and The transmitting portion TSE comprises inorder an amplifier A7 having its input bridgedacross the outgoing pathWA on the output side of amplifier'As, a band-pass filter F3 and thewave detector D3. The output of detector D3 connects to the loweroperating winding. we of the differential relay DB2, .and

through delay circuit D02 and hang-over network HNs to the switchingcontrol e2. The receiving echo suppressor portion RISE includes inorder, the amplifier As having itsin'put connected across the incomingportion of the path EAin front of amplifier A2, the bandfilter F4 andthe detector D4; The output of detector D4 is connected in parallel tothe upper operating windinw ws of'diffe'rjential relay DB2, poledjtooppose winding, 104, and through the hangover network HNi in parallel.to the fourswitching controls 112, baczand The differential realy DB2controls the two- 'way stepping switch SS2 which controls the adjustmentof variable loss R2.

The function. of the differential relays, the

various switching controls and. the other apparatus of Fig. 1 will bebrought out ininore detail in the following complete description ofoperation. V

Let is be assumed that the westsubscriber associated with the line LWshortsspeaking. His

speech wave Will dividein hybrid coil Hw and the useful part will passupward into the west-to-ea'st transmission path EA, and will betransmitted over that path through amplifiers A1, A2, hybrid transformerHE and linefLE to the east subscriber. The loss control P1 will not beeffective to insert any loss in the transmitted Waves because control.a. being in its normally unop-;

erated conditio'nQPr is short-circuited. I v I A portion of wests speechwaves at the west terminal will be diverted into TSw and pass throughbridged amplifier am band-pass filter F1 and will divide between thewinding wi of differential relay DR; and the inputof delay circuit Thedelay circuit DC 1 will delay the trans-, mission of the speech wavesfor a time T, whereT is substantially equal to the transmission timewire circuit. Thus, at the "same instant that eter P2 is controlled byth amplifier As, band-pass filter F4 and detector D4 to energize theupper winding we of differential relay DB2, and through hang-overnetwork l-IN;

"will. cause .the operation of switching controls (12, b2, 02 and d2.Control a2 operates to insert the part of the potentiometer resistanceP2, de-

termined by the setting of its variable arm, into the outgoing portionof the path WA at the east termihalto attenuate the echoes of westsspeech currents returned from the east subscribers line "llEjthrb lghhybrid cpiin he e ti ior the 'variabl] arrnioif gbgm p n' switch SS2,thecperation of which is cont, olle'd'by differential 1 over the pathEA between terminals of thefour- 'scriber.

relayIlRz, which in turn. depends on the relativedegreejlof,energization of, its two windings w: and i014. WindiI .S.'ws'and jwrare energized respectively bythe incomin'gnvoice currentin EAIthrough ,the receivingfsuppressor 'RSE, and by the outgoingor echocurrentsin WA transmitted intothe transmitting suppressor TSm Thedifferential relay'DRz cannot cause any adjustment unless the receiving,suppressor RS'Eis operated .to causefthe operation 'o'fcontrol dz to,connect battery 'B2 to the relay armature, The ,irelay DB2 may eitheroperate its armaturejtothe right or lef CQ ltact or sta'y in its,neutral position. It

will operate, to the right contact iif the incoming speech wavessupp-lied to the upper winding w; of

the relay areutoo-'stro"ng. ;in relation to the echocurrentsuppliedtofthe lower ,r'elay winding 1.04,

and to the leftlcontact if the echo is 'to'ostrong with relation 'totheincoming speech waves The armatlite of the relay DRz will remain in "theneutral. position if the echois correct with regard totheincomingwave,Operation of relay DRz to the right will cause theloss in Pzftodecrease,

as indicated by shortiria'dut, more or "less of the loss as the case maybe "and operation in the left direction will cause the loss provided byP2 in the path WA toincrease. I

: Controls b2 .andcz operate in 'responsejto the incoming speech. wavesat the east terminal, to

disable jthe transmitting suppressor TSE at the V input andoutpht jotdelay circuit 'DCz, which has a 'delay of T, respectively. This ,isnecess'ary to prevent operation of control (e2 by echoes and also to'givefc'ontrol to -the west. subscriber .durin'g double talking provided,he spoke first.

Thus,.let it be, assumedthat, the west subscriber started to talkslightly, before [the leastfsub- Theispee'ch waves of, both would getstarted over the respective e ast.-towest 'and west-to-each speechtransmission Q path without undergoing any attenuation by 'thepotentiometen re'sistarice at the, near terminahand the divertd'speechenerg or west jand'e'ast at the west and east terminals will be startedthrough N delay circuit, DCI and D02, respectively. However, since'wests'tarted ,talking f rst; hisjspe ch wave wouldvarriveiatvtheeastend of the fourwire Lcirc'iiit ,beioreueastfs speech impulse gets.through'the 'dflay circuit .DQz, and also before theses;jsub'scriberswavefgets'ftothe west end of the four wirecircuit. Controlsbz and at -fier A1 a part of wests outgoing waves will be the eastterminal would then operate to prevent operationof control e: by east,and would also wipe off any speech impulse started through delay circuitDC2 by east Control e1 at the west end of the circuit would also beoperated by west to disableRSw, thus preventing operation of any of thecontrols at the west terminal by easts speech when it arrivesat the westterminal. A small spurt of easts speech would, however, get through tothe west subscriber Without attenuation, and if both subscriberscontinue talking later, later speech from east would arrive at the westterminal attenuated by whatever value of loss which will be inserted inthe westto-eastf path WA by potentiometer resistance P2.

It is apparent that breaking would be made appreciably easier by thearrangement of Fig. 1 than'in the usual echo suppressor which alwaysputs in a fixed amount of loss which is large enough to suppress echoesunder the most severe conditions. Thus, when the return losses are good,the suppression loss provided by the variable loss potentiometer devicesP1 and P2 might be quite small, and the action of the circuit would notbe Widely different from that of a circuit with no echo suppressor atall. Only when the return losses are quite poor would the loss have tobe as large as it is at all times with most present echo suppressors.

Continued double talking would tend to operate both differential relaysDB1 and DR2 in the direction to increase the suppression loss at bothends of the circuit. Normally this loss would only have to be adjustedonce during a call, however, and thus it is believed that the lossadjustment could be made moderately slow, say, to require two or threeseconds to get over its total range. In this case, with the doubletalking which ordinarily occurs in such circuits, the change in thesuppression loss would ordinarily not be very large during doubletalking and, at any rate, the loss would be adjusted back to its regularvalue a short time after the double talking had ceased.

Fig. 2 shows the detailed circuit arrangement which could be used ateach terminal of the system of Fig. 2 to accomplish the functionsdescribed above for Fig. l. The west terminal of the circuit isillustrated and the corresponding transmission apparatus circuits aredesignated with similar identification characteristics. The circuitdetails will be brought out in the following detailed description ofoperation of the circuit of Fig. 2.

' -Let it be assumed that the west subscriber connected to the line LWspeaks. His speech waves will divide in hybrid coil Hw, partbeingdissipated in the output of amplifier A4 and the useful part passinginto the input of the westto-east path E'A. Wests speech currents willfirst pass through the loss pad L1, the function of which is toeffectively remove from the twowire line the efiects of the varyingimpedance of potentiometer resistance P1, and then will pass through thetwo repeating coils T1 and T2 and amplifier A1 out over the path EA tothe last terminal of the four-wire circuit. If the receiving suppressorrelay RHO1, corresponding to the control (11 in the system of Fig. 1, isnot operated, the potentiometer P1, the resistance setting of which iscontrolled by the two-way stepping switch SS1, comprising the twostepping magnets f1 and f2, will be shorted out and there will be nosuppression loss applied to wests outgoing speech Waves, At the outputof amplidiverted into the transmitting portion TSwof the echo suppressorat the terminal and will be amplified by bridging amplifier A5 andpassed ing 201 of the difierential relay DR1 and, if strong enough, willoperate relay TM1, and will produce a pull on the relay DR1 tending tomove its armature to the upper relay contact. Let it be assumed thatrelay TM1 is operated. This removes the ground from condenser 02 throughresistance R2 and then allows relay DM1, normally operated by currentfrom battery B3 through resistance R3, to start to release. The relayDM1 and the associated circuit perform the function of the delay circuitD01 in the circuit of Fig. 1. After the desired time interval, T whichis equal to the one-way transmission time over the four-wire circuitbetween terminals, and may be controlled by proper proportioning of thevalues of condenser C2 and resistance R3, relay DM1 will release,thereby operating relay THO1, corresponding to control 61 in the systemof Fig. 1, almost immediately due to the discharge of condenser C3through R1 which is comparatively small. Relay THO1 will be operated bycurrent from battery B4 transmitted through its winding and resistancesR5 andRi. Operation of relay THO1 immediately disables. the receivingdetector D2 in its input, thus preventing its operation by echoes fromthe distant end of the circuit. (With this arrangement, there willalways be some echoes, and these echoes may be relatively strong in thecase of a very strong local talker.) Relay THO'1 is given sufiicienthang-over by the combination of C3 and R5, corresponding to hang-overnetwork HN1 in the system of Fig. 1, so that it holds the receivingdetector D2 disabled for a time sufiiciently long to take care of allthe delayed echoes.

The differential relay DR1 will probably be operated very frequently inthe up direction by the local talker, but this will have no effect onthe stepping of potentiometer P1, since relay RHO1 will not be operatedat such times, and thus there is no ground on the armature of thedifierential relay DR1.

Let it be assumed now that wests outgoing speech has ceased, andincoming speech waves from the eastsubscriber are present in the pathWA. A portion of these waves will be diverted into receiving portion RSwof the echo suppressor and will be amplified by A6, filtered by F2 anddetected by detector D2 which is now operative due to the release ofrelay THO1. The pulsating direct current in the output of D2 will besmoothed by condenser C4 and will tend to cause a pull in the downdirection on relay DR1 and windings and resistances R6 and B1 frombattery B4.

The operation oi'relay RI-IO1 immediately'inserts loss in the path 'E-Aby removing ground from the arm of potentiometer P1 thereby removing theshort from the potentiometer. When the armature of RHO'i reaches itslower contact, it'will also put ground on the armature of thediflerential relay DB1 and thus allow adjustment by the stepping magnetsf1 and f2 under control of differential relay DB1. Operation of relayRHOz immediately discharges condenser C2 through resistance Rs. Thus,even though the release of relay DM1 has beeninitiated by'a previousoperation of relay TM1, this action will be completely annulled byoperation'of relay BHOz.

The differential relay DB1 will now'be affected by incomingwaves'through winding wz and echo Waves attenuated by the loss providedby the variable resistance of potentiometer P1 through winding 101. Ifthe 'echo wave amplitude is too high compared to that'of the incomingspeech waves, the settingof the differential relay DB1 will be such thatits armature will operate in the up direction. This will cause thepotentiometer to step in the clockwise direction thereby increasing theseries resistance in the path EA and thus increasing the loss in thatpath. When the loss is sufficiently increased by successive operations,the currents in w1'and wz will become equal and there will be no furtheradjustment since the armature of DB1 will then remain in the center(neutral) position during' incoming speech waves.

Now, let it be assumed that'a new call comes up in which the return lossis considerably better than that on the one just previous. In this casethe loss due to the potentiometer Piwill be quite high, and duringin'comi'ngspeech the current in winding wz of relay DB1 will beconsiderably greater than in winding wi. This will cause operation ofthe differential relay DB1 in the down direction and willstart'a seriesof stepping operations in a counter-clockwise direction inpotentiometer'Pi. Thiswill continue until the echoes at the input ofamplifier A5 are increased sufficiently so that the'armature ofdifferential relay DB1 once'more stays in the'neutral position.

As noted abova'doiible talking while it lasts will produce an eiiect onthe differential relay DB1 which is similar to loud echoes'and thus willprobably operate it in the up direction and increase the suppressionloss. By making 'the stepping switch slow oper'ating byproper design,

however, this effect canbe made unobjectionable. Another way of 'makingthe ,effectunobjectionable would be to design thestepping switch so thatit would not operate'at' all'for very brief pulses. Actuallymoststepping "switches orselectors are such that they do not operate onvery brief pulses anyway.

Various modifications of the circuits which have been illustrated andde'scrib'edwhich are within the spirit and scope of the'invention willoccur to persons skilled in the art.

What is claimed is:

1. In a two-way "signal transmission system including a two-way signaltransmission path, one one-way'signal transmission path for transmittingto said two-way path'and a second oneway signal transmission path fortransmitting from said two-way path, an echo suppressor operating "inresponse'toincoming signals insaid one one-way path to insert "anechosuppression loss in said second'one waypathj means to autotrolled 'bythe cdrnparingmeans to automatically adjust the value "of said echosuppression loss *so that its magnitude is just sufiicient to'properlysuppress said returned echoes.

;2. The system oi claim l,'in which said comparing means compares thelevel of the signals in lsaid one path with the level of the signalechoes said-second 'path' beyond the point at which said echosuppression loss in inserted "therein, said adjusting means iscontrolled by said comparing means to increase the value of saidlsiuppr'es'sion loss to the required degree when the level of the'echoes'transmitted through said suppression loss is toolarge withrespect to the le ve'l of the received signals in said one path, todecrease the value of said suppression loss when the level of the echoestransmitted therethroughis too small with respect to the level of saidreceived signals, and to make no adjust- 'ment of the value of saidsuppression loss when "the comparison of the signals and echoes by saidcomparing means indicates that the suppression loss is of the correctvalue.

3. In a two-waysignaltransmission system, a two-wire line, a f our-wiresignal transmission circuit connected thereto including one one-waycircuit for transmitting to said line and a secondoppositely-directedone-way circuit for receiving from said "line, anecho suppressor including a portion connected to said one one-Waycircuit responsive to incoming signals therein to introduce. an echosuppression loss in said second one-Way circuit, control means forautomatically varying the value'of said echo suppression loss, duringthe period in'which said portion of said echo suppressor is operated byincoming signals,

so'that the value of saidloss is'increased when the ratio of the levelof the signal echoes in said one one-way circuit with respect to thelevel of the incoming signalsin said second one-Way circuit is greaterthan a tolerable value and the 'value of said loss is decreased when theratio is less than the tolerable value.

4. The'syst'em of claim 3'in which said echo suppressor includes. asecondportion connected to saidsecond one-way circuit at a point beyondsaid suppression loss, operating in response to outgoing signals in saidsecond circuit but with a fdelay substantiallyequal to the overalltransmis- "sion time oversaid second "one-way circuit between theterminals of said four-wire circuit, to

disable the first portion of? said echo suppressor.

5. In "a two-way signal transmission system including a two-waytransmission circuit, one one w'ay'path for'transmitting from saidcircuit and a'second- "one way path'for transmitting to said "circuit,means responsiveto signal transmission'betwee'n said second path andsaid circult for inserting an echo suppression loss in said one path,auxiliary means responsive to one given ratio of the level of thesignalwaves in said second path to' the level of the signal echoes returnedfrom-said circuit to saidone one-way path to increasethe amount ofs'aidsuppression loss, and

responsive to another given lower ratio of the level of the signal'wavesin said second path to the'level of the echoes in'said one path toreduce the" amount of said echo suppression loss.

6. The system "of claim'5 in which the coupling between said two-waycircuit and said one and said second path comprises a hybrid coil and anassociated balancing network, which provides an approximate conjugaterelation between said one and said second path, and said auxiliary meanscomprises a differential circuit for automatically comparing the signalsin said second path with the signal echoes in said one path during theperiod in which said signal responsive means is operated, and meansoperating automatically to increase or decrease the value of saidsuppression loss according to whether the echoes are too strong or tooweak relative to the signals in said path.

7. In a two-way telephone system including a two-way transmissioncircuit and incoming and outgoing one-way signal transmission pathsconnected in approximate conjugate relation with each other and inenergy-transmitting relation with said two-way circuit, an echosuppressor comprising a receiving branch connected to said incomingone-way path, responsive to incoming telephone signals therein to insertan echo suppression loss in said outgoing one-way path, and atransmitting branch connected to said outgoing one-way path, responsiveto outgoing telephone signals therein to disable said receiving branch,a difierential circuit controlled from said outgoing and incomingone-way paths for automatically comparing the signals in said incomingpath with the echoes in said outgoing path beyond the point of insertionof said echo suppression loss therein, during a period in which saidtransmitting branch of said echo suppressor is operated, and meanscontrolled by said difierential means for automatically adjusting thevalue of said suppression loss so that it is never greater than isnecessary to properly suppress the echoes.

8. The system of claim 7 in which said diiferential means comprises adifferential relay having two opposing windings, means for energizingone winding in proportion to the level of the signals in said incomingone-way path when said receiving branch of said echo suppressor isunoperated, and means for energizing the other winding in proportion tothe level of the echoes transmitted through said suppression loss insaid outgoing one-way path, said relay operating in one direction whenthe signal level exceeds the echo level by a given amount indicating toogreat a suppression of echoes by said loss, and operating in theopposite direction when the signal level exceeds the echo level by agiven lower amount, indicating too little suppression of echoes in saidloss, said means controlled by said diiferential means comprises atwo-way stepping switch operating to decrease the value of said echosuppression loss in response to operation of said relay in said onedirection until said relay releases, and to increase the value of saidsuppression loss in response to operation of said relay in said oppositedirection until said relay releases.

9. The system of claim 7, in which the echo suppression loss inserted insaid outgoing one-way path by operation of the receiving branch of saidecho suppressor is a potentiometer resistance, said resistance beingshort-circuited when said receiving branch is unoperated so as to beineffective in said outgoing one-way path, and said means forautomatically adjusting the value of said suppression loss under controlof said differential means comprising a stepping selector switchoperating to adjust said potentiometer resistance in a direction toincrease the insertion loss in said outgoing path when the comparison bysaid differential circuit indicates that the loss is smaller than thatrequired to properly suppress the echoes, and operating to adjust saidpotentiometer resistance in a direction to reduce its insertion losswhen the comparison by said differential circuit indicates that the lossis greater than is necessary to properly suppress the echoes.

10. In combination with a four-wire telephone circuit interconnectingtwo two-way telephone circuits, said four-Wire circuit comprising twooppositely directed one-Way transmission paths for respectivelytransmitting telephone signals in opposite directions between saidtwo-way circuits, echo suppressor means at each terminal of saidfour-wire circuit comprising a voice-operated receiving switching branchconnected to the incoming one-way path and responsive to incomingtelephone signals therein to insert an echo suppression loss in theoutgoing one-Way path at the terminal, a voice-operated transmittingswitching branch connected to said outgoing oneway path and responsiveto outgoing telephone signals, but with a delay substantially equal tothe oneway transmission time between terminals of said four-wirecircuit, to disable said receiving switching branch, a differentialcircuit operating automatically to compare the level of the incomingsignals in said incoming path with that of the echoes of the signalsbeyond the point of insertion of said suppression loss in said outgoingpath and means controlled by said differential circuit to increase thevalue of said echo suppression loss when the echoes are too strong andto decrease the value of said suppression loss when they are too weakwith respect to the incoming signals.

11. The combination of claim 10, in which the delay in the operation ofsaid transmitting switching branch is attained by the use of a delaycircuit of the required value in that branch, said receiving switchingbranch when operated also disabling said transmitting switching branchon each side of said delay circuit and conditioning said diiferentialcircuit for operation.

DOREN MITCHELL.

